1. Field of the Invention
The present invention relates to a projection optical system, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
Japanese Patent Application No. (“JP”) 10-242048 proposes a method for controlling an aberration of a projection optical system in an exposure apparatus by shifting each of a pair of optical elements (in an Alvarez lens) in a direction perpendicular to an optical axis, where the pair of optical elements have complement aspheric surfaces facing each other and are separated by a fine gap in the Alvarez lens.
However, in JP 10-242048, the fine gap between the pair of optical elements causes an aberration. For example, the gap of 100 μm between the pair of optical elements causes non-negligible amounts of distortion and astigmatism but it is difficult to further reduce the gap between them. In other words, the conventional exposure apparatus using the pair of optical elements causes an aberration and deteriorates the overlay accuracy.